Bug 1352113 - Shippable Builds - Don't run OSX opt builds by default on integration trees. r=aki

Differential Revision: https://phabricator.services.mozilla.com/D23128

--HG--
extra : moz-landing-system : lando

taskcluster/ci/build/macosx.yml

@@ -64,6 +64,7 @@ macosx64/opt:
         script: "mozharness/scripts/fx_desktop_build.py"
         secrets: true
         tooltool-downloads: internal
+    run-on-projects: ['mozilla-central', 'try']
     toolchains:
         - linux64-binutils
         - linux64-cctools-port